


Strap-in, Baby

by wearyeyebrow



Category: Undertail - Fandom, Undertale (Video Game)
Genre: Consensual Sex, D/s undertones, Demisexual Sans (Undertale), Ecto-Vagina (Undertale), Established Relationship, F/M, Implied Dom Reader, More Humor Less Sex, Oral Sex, Slice of Life, Strap-Ons, Sub Sans, Vaginal Fingering, Vibrators
Language: English
Status: Completed
Published: 2017-03-30
Updated: 2017-03-30
Packaged: 2018-10-13 00:09:51
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,332
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/10502355
Author URL: https://archiveofourown.org/users/wearyeyebrow/pseuds/wearyeyebrow
Summary: It’s strap-on time, and guess who’s getting fucked?I bet you’ll never guess. (PSA: it’s Sans)But not before some trial and error involving good old fashioned shenanigans.





	

It’s a quiet Thursday afternoon when the doorbell rings. You leap off the couch, almost spilling your coffee, giddy with anticipation – you’ve been tracking this package since last Tuesday, and it finally went out for delivery.

You hear a thump as the UPS driver walks away. They’ve left an average brown box on your doorstep, the word ‘novelty’ stamped on the side. 

You grab some scissors and head to your bedroom. Inside is an absurd amount of brown wrapping paper, and a packaged pair of black underwear. They almost look like men’s briefs. You’d think that’s all they were if not for the hole in the front, and the thick material surrounding it.

Yeah, it’s not the sexiest one you could find, but you don’t want to fiddle with straps, or worry about it falling off. You’re more about utility.

Your pants are flung to the other side of the room as you hurry to try it on. It slides over your body like a glove. You glance at yourself in the mirror; nothing special, just you in slightly masculine underwear. But that’s not all these are. 

You cackle like a madwoman before catching yourself. Calm down. You’re excited and anxious to give it a shot. 

After all you’ve used sex toys plenty of times with him, dildos included. You know exactly what he likes. 

You’ve just never had one attached to you, nor have you ever been in this position before. You feel like a virgin that’s watched a lot of porn in their spare time. You know what to do and how to do it, but what’ll actually happen is an entirely different story.

You crawl over your comforter, reach for the box under your bed, and grab your dildo of choice. It’s black and made of smooth silicone. It looks the least like a sex toy of all your sex toys. You’ve contemplated setting it next to your cactus collection and labeling it a work of modern art. Sans said to placard it with the title ‘Rod of Glory.’

You scrapped the idea.

It fits into the metal ring with little trouble, and after a bit of adjusting you turn around to check yourself out in the mirror. 

There’s a beat of silence before you burst out laughing. 

You give a test thrust and your mouth sounds reach new decibels, as Papyrus would say. 

‘It-it’s so – it’s like I have a spear, fucking attach a knife to this thing, it’ll getcha!’ All you can think of are horror movies, imagining dramatic, creepy music playing as you pelvic thrust your way into a room.

You’re in tears when you hear a key in the lock of your front door. You double over, sliding to sit on the floor, your back against the side of the bed. 

You’ve almost caught your breath when you hear a gentle knock on your bedroom door. ‘sounds like a party in there, mind if i join in?’

‘I mean you can, but do you want, to is the real question,’ You wheeze.

‘well now you’ve just piqued my curiosity.’

‘I invite you to satiate your interest.’

The door swings open and the first thing he sees is the brown wrapping paper on your bed, lying in clumps. There are scissors, a box, your pants, and then there’s you, sitting on the floor, legs spread, gesturing and wiggling your eyebrows at the black object of your elation. 

‘I don’t know whether to feel sexy or lament the fact that my limbo game is ruined.’

‘how low can you go?’ he deadpans.

‘Pretty low.’ You give him a wink, and throw in some gun shots.

He cracks, laughing as he gives you a once over, ‘i mean… if you wanna try, i’m down.’

‘What about now?’

He thinks for a minute, then shrugs ‘sure, i guess.’

‘Thanks.’ You stand up, trying not to laugh as the dildo bobs, ‘But um, as excited as I am to try this, you know I’ve done some reading, and I can’t promise I’ll give a good… performance.’

His fingers brush against your waist, ‘then don’t,’ he winks, before shuffling over to the other side of the room and hanging his jacket on your bathroom door.

-

Twenty fantastic minutes later and you have a hand down his shorts, fingers rubbing at his pubic symphysis. He fumbles at his waistband, before kicking his shorts to the end of the bed. The second you remove your hand a soft crackling sound permeates the room. It excites you more than you think he realizes.

Beginning at his pubic arch, a manipulation of energy forms, taking shape. After it settles the material feels almost flesh-like, smooth, entrance dripping with milky liquid. But it’s alien in color, the anatomy a little off. You’ll admit it was odd when you first realized you could see his cunt from both directions. But the premise is the same. His clit is still just as sensitive, and his g-spot still just as hard to find, but also just as rewarding. And he tastes like nothing else, a completely unique flavor that makes your mouth water. 

He’s propped up on one arm, breathing a little off as you drag your fingertips over his cunt, soft and teasing.

You thumb his clit, paying attention to his subtle changes of expression. You reach forward and lazily jerk his spine. He falls onto his forearm, hand subconsciously over his mouth, eyelights following your every move. You slip a finger inside of him. Any noise he makes hard to catch behind his hand. You stay there and add another finger, slowly massaging his g-spot. 

His head falls back onto the pillow, hips trying to establish a shaky rhythm.

‘should, uh, probably switch soon, if you wanna try this out tonight.’ He murmurs breathlessly. 

You’ve gotten a little distracted, loving the way he rocks his pelvis against your hand.

‘Probably…’ You’re tempted to fall back on tried and true methods, forget this potential train wreck and go for gold. You exhale through your nose, and slowly withdraw your fingers, before snagging the dildo from your nightstand, snapping it into place.

He chuckles breathlessly as you slather it with lubrication, ‘you seem impatient – does that mean you’re doing a… jiffylube?’

You pause to keep your composure, watching as lube slides down your silicone cock. ‘Don’t give them free advertisement.’

‘i’m sure necrophilia sells real well.’

‘Then call me a necrophiliac, because I’m sold.’

His cheek bones darken, ‘…your timing was solid. 9/10. great line.’

‘It was a team effort, much like this will have to be.’ You stare apprehensively at your new appendage. 

‘what do you want me to do?’

‘Talk to me. Tell me if it hurts, or if you want me to change something.’

‘k.’

You concentrate, going over what you’ve read. ‘Spread your legs.’

‘you could at least say please,’ he jokes, adjusting his body.

‘Heh, sorry,’ You lean forward, hunger in your eyes, desire in your voice, ‘Please, Sans?’

‘well, when you put it like that, why would i wanna say no?’ He mumbles.

-

‘uh, maybe you could…’

‘You’re gonna have to be explicit.’

‘…it’s bordering on uncomfortable, like, pull out a bit, adjust the angle, something.’

‘Got it.’

He chuckles at your tone, ’10-4 sergeant.’

‘This sergeant should be demoted.’ You say as you ease out of him a bit, wishing you could feel something, anything, except for the dull ache of your pubic bone. 

‘nah, she’s doing just fine. gotta cut herself a little slack.’

You sigh, ‘Point taken. Tell me if you need more lube – I can’t feel with this thing.’

‘roger.’

The muscles needed to move your hips properly are weak, and you struggle to find the right angle. You wish you could feel where you’re at inside of him just like you can with your fingers. But the only thing you can feel is the metal fastener on your harness painfully pressing into your pubic bone every time you try to move. 

And he teases you more than once for your detached focus as you figure out the ‘logistics of his body from a new perspective.’

‘makes me sound like a map.’

You glance over his rib cage, ‘That’s a good way to describe you.’

‘am i the trodden path, or does that come later?’

‘I… Sans.’

‘will i get larger if you unfold me?’

‘Sans-’

‘i’d say you’re in the southern quadrant.’

‘Sans!’ You can’t help but laugh as you slump forward, your forehead against his sternum. The deep rumble of his laughter reverberates through his rib cage.

‘I’ll figure this out, somehow.’

‘i know.’

‘Put up with my serious face until then, yeah?’

‘nah.’

‘C’mon, I’m new at this.’

‘i know.’ He laughs softly, ‘i already like parts of it. sides’, it doesn’t have to happen right away. i mean,’ he rubs the back of his neck, ‘it didn’t exactly go to plan the first time we did it.’

You smile fondly, ‘Yeah, but that’s why I loved it.’

‘i know,’ his cheekbones darken, ‘so, cut yourself some slack. s’not a big deal that you don’t have it figured out.’

‘Thanks.’

‘you could even say it’s… harder than it looks.’

You can only laugh weakly, your strength waning. You’re used to slamming against bone, not getting your own bones slammed. Even though you’ve ridden him just fine, your legs are already shaking. 

‘you okay?’

‘I don’t think I can go much longer… I’m not in the best shape of my life. I just wish I could find the right angle.’ You sigh.

‘s’ok, we can try again later.’

‘You’d be willing to go through this again?’

‘think i can take it.’ His phalanges softly trace over your waist, running up your rib cage, thumbs underneath your breasts.

‘Thanks, but don’t think I’m going to leave you hanging.’ You slowly ease out of him.

You stand up on wobbly legs and slide the apparatus off, setting it on your dresser like a forlorn mountaineer looking over the horizon, and lay flat on your stomach between his femurs. 

Even if the strap-on was a failure, the fingering sure wasn’t. His folds are puffy, glistening with translucent cum, clit still swollen. ‘Mmm… Thanks for having me. Dinner looks great.’ You say as you run your nails over his hipbones. 

There’s a second of laughter before your mouth is on him and he’s unable to think clearly.

-

Your efforts over the next few weeks have given you mixed results. It works, just not well enough. You’ve got it down, but you can’t go for very long. You’ve found yourself daydreaming at work, trying to come up with ideas on how to make it more doable. 

Sex with him is always fantastic, toys or not. You’d be fine labeling the strap-on a wash, if not for your stubborn streak. Even if it ends up dusty in the back of your closet, you still want it to work at least once.

That’s when it hits you – sex toys. You’ve got others hidden away in your closet, under your bed. Who says you have to use just one at a time?

You’re sitting on a crowded city bus, legs crossed on your way home from shopping. Straight-faced, trying to think of which toys to use, and how best to use them together. An elderly woman steps on and you give up your seat. Maybe these thoughts are better saved for an evening in. At home.

You grab the only free handle, forcing you to stand with your face in a man’s armpit. 

On second thought, maybe a little distraction is a good thing.

-

A few days pass without seeing him, your schedules just not lining up. But you can usually count on Friday nights, and this week’s no different. He shows up to watch some B-horror films, and get popcorn stuck between his teeth. You eat some stir-fry you threw together, and then you get an annoying email from work, demanding you reply. He reclines on the other side of the couch, making you laugh with fake snoring noises that get more absurd as time goes on

Half an hour later, and you snap your laptop closed a little harder than intended, ‘Done.’ You glance over at him. He appears half asleep.

‘Sans.’

‘hmm?’

‘You awake?’

‘no.’

You lean back against the armrest, ‘My bed is more comfortable than the couch.’

He opens one eye socket. ‘is it?’

‘You should know, given how many times I’ve had you in it.’

‘you got me.’ He sits up, joints popping. 

‘Are you willing to stay up with me tonight, is what I’m asking.’

He chuckles, looking to the side, ‘pretty sure you already know the answer.’

‘Just checking. C’mon,’ you give him your hand, ‘I’ve got a few things planned, if you’re up for it.’

He raises a brow bone and takes it, following you to your bedroom.

You close and lock the door behind you with a soft click, before pushing him up against it, hands grasping his hipbones. He unzips his jacket and shrugs it off, letting it fall to the floor.

This is where you wish you could kiss him, tongue in his mouth, biting his lip – but that’s one thing he can’t do. You’ve learned how to manipulate the tiny bones of his neck instead, simply loving the small, involuntary noises he makes. The way he presses against your body, hands gliding up your shirt over the expanse of your back. These soft, open responses light your body on fire, your differences forgotten. 

‘Mmm, shit,’ you groan, ‘I’ve been waiting for this all week.’ 

‘yeah?’ He asks, breathless.

You pause, pressing your forehead against his shoulder. ‘Do you mind if I try the strap-on again?’

‘s’ok by me.’

You breathe out, ‘Thanks.’ 

You pull away and head to your closet while he sits on your bed. You rummage through it, grabbing everything you’ve prepared.

You drop your pants and underwear. ‘I’ve been thinking…’ you snap the harness on your hips, ‘That I need to approach this from a different angle. So, I’m gonna try something more my style, I guess.’

‘your style?’

‘Mhmm.’ You set a vibrator on your nightstand, along with the dildo and a bottle of lube. 

‘ah,’ he chuckles, ‘that looks like you.’

‘Maybe it’ll go a little better this time,’ you smile wryly.

‘it was fine the first time.’

You blow hair out of your face, ‘I know. I just... don’t wanna disappoint you,’ a hand on the back of your neck, ‘And I know, I know you’d never say it that way, maybe never think of it that way but… I can tell when you aren’t satisfied.’

‘do i need to break out a math analogy?’

‘Huh?’

He shakes his head, ‘but that doesn’t matter. besides, i can’t get into it unless you are. don’t think so much. that’s my job.’

You breathe out softly, ‘Yeah. You’re right.’

‘it’s not about being right,’ he interjects, ‘i just… you’re – no matter what we do, it’s the fact that it’s you, that’s important. what we do doesn’t matter as much.’ He clears his throat.

Oh. Right. This isn’t a performance. Your nerves fade, settling underneath the bed as you slide over him, forcing him back until his head hits the pillow. 

Your lips find his jawline as you sink into his body. ‘I love you. I hope you know that.’ You say softly, slipping your tongue between his cervical vertebrae. He breathes in sharply through his nasal cavity.

You don’t say it often, but sometimes it feels like the only appropriate thing to say.

‘love you too,’ he murmurs, fingers tangling in your hair.

There’s an easy affection between the two of you that slows your train of thought. You almost forget about your original purpose until your thigh accidentally presses between his legs, and he lets out a quiet sound that goes straight to your groin. You smile and slip your hand down his shorts, fingers teasing the holes of his sacrum.

‘Don’t think I’ve forgotten,’ you whisper into his temple, ‘Let’s take our time.’ 

-

‘you said you were going to…?’

‘Mhmm, in awhile.’

‘awhile?’ He breathes.

You’ve got the vibrator going, lowest setting, rocking back and forth on his clit. You’ve straddled his legs, your weight holding him down.

He’s panting, toes scrunched, an arm over his eye sockets

‘if you- if you keep,’ he whispers.

‘Not yet.’ You remove the vibrator and he pushes his hips against you. You run your fingers over his body, nails softly scratching between his ribs.

His breath hitches at your hands on his pelvis, sighing, eye sockets hooded. 

You turn the vibrator up a notch and put it back, savoring how he gasps, eyelights rolling in the back of his head, spine arching. ‘jesus christ…’ 

‘You wanna come, Sans?’

‘but-’ 

‘Don’t think about it, don’t worry about it,’ you emphasize, your voice dropping in pitch, ‘Let me take care of you.’

You slip your other hand between his legs, easily getting two fingers inside of him. He shifts his pelvis against your fingers, hand going over his mouth when you crook them just right.

‘C’mon, let go for me.’ You murmur.

‘oh fuck.’ He jerks when you turn the vibrator on high, clinging to the sheets, body trembling beneath you, hips trying to push into your hand. 

You massage his g-spot, feeling the ache between your legs throb at the feeling of his cum covering your hand. As you move faster, some of it ends up on your thighs. He lifts his hips, legs tightening, and you struggle to maintain the right angle and keep the vibrator still.

‘yes fuck, there-!’ He tries and fails to match the speed of your fingers; he’s left writhing beneath you, fists clutching the sheets.

He’s so close. You keep it up until he comes, arching his spine and growling through his nose, fabric straining under his grip. 

You ease the vibrator down before removing it, and scoot back between his femurs, spreading his legs. He jerks with every swipe of your tongue over his sensitive clit, moaning your name as you get into it. You lick your lips when you pull away, leaving him shuddering, breathing hard, trying to concentrate on what you’re doing – you grab the dildo and slip it into place. 

He weakly reaches for your hips, hands shaking.

You squirt some lube into your hand and coat it before rubbing the tip against him. You go slow, meeting very little resistance, slipping it inside of him, inch by inch. You shift on your knees between his legs, and adjust your hips. It slides in further until you’re seated as far as you can go. 

‘M’gonna move,’ you murmur. He lets out a breathy moan as you start pulsing your hips. Your free hand scrapes that spot between his lumbar vertebrae.

He struggles to catch his breath, ‘_____, it’s so – fucking hell,’ his phalanges dig into your skin.

You keep moving, breathing through your nose. ‘If you want me to stop you’ve just gotta say the word.’

He shakes his head, falling back onto the pillow. ‘no that’s not… fuck.’

You brace yourself with an arm, and lean down to kiss his neck, you don’t expect him to grab the back of your shirt and keep you there, hands fisting the fabric, fingers digging into your skin. You can hear the little noises he doesn’t realize he’s making, tiny whimpers with each shallow thrust. Your breath is hot against his shoulder, ‘That’s it baby boy… let me make you feel good.’ 

Your back starts to ache from the movement, nylon digging into your inner thighs, but it’s a minor distraction as he rakes his phalanges down your back, his grip on your shirt nearly choking you. 

‘yes, fuck yes, _____’ He whines your name before burying his face in the crook of your neck, clenching his jaw. But you just can’t move fast enough, can’t quite get there. He’s so close, you have to speed up. 

You slam your pelvis against his, deep inside of him, and grab the dildo with your hand. There’s enough leeway in the harness that you can fuck him as if it wasn’t attached, like you normally would, wrist cramping with how fast you’re going. He reaches between the two of you and frantically rubs his clit, ‘yes, yes, yes- fuck me-’ he braces a hand against the headboard.

You growl, sweat dripping from your forehead.

His voice climbs in pitch. A few more seconds and he freezes, legs squeezing together, body tightening. He lets out a choked sob, his phalanges digging into your skin leaving little crescent moons scattered all over your back. 

You keep up the pace, and his voice shakes, tone getting desperate. He whimpers beneath you, ‘fuck, fuck, fuck i-i can’t, baby please, it’s-it’s-’

You slow, your pace going from hard and fast to deep and slow as you roll your hips against his. ‘oh my god…’ 

He whispers, voice hoarse, ‘this is - don’t stop, please don’t stop.’

You snag the vibrator and flick it on, jamming it between the two of you. You can feel it through the harness, stimulating your engorged clit. 

‘Fuck,’ you croak, throat dry. You try and keep the vibrator still with one hand, but can’t quite seem to manage, until he takes it from you, keeping it there and freeing your hands. You fall onto your forearms, hands fisting the sheets, still fucking him with your hips, slow, methodical thrusts as you jerk at the intensity of the vibrator.

He gasps beneath you, little ragged sounds as his phalanges run softly over your back. 

‘so good,’ he moans, ‘so – holy shit.’

Every time you thrust the vibrator is forced into your clit, the stimulation agonizing. It’s almost enough to make you come.

‘S-Sans, baby, I… I’m-’ he can hear the desperation in your voice. 

‘you close?’ His voice cracks. You can only nod into his shoulder. ‘please baby,’ he whimpers, ‘please, fuck i wanna see it, wanna see you come, wanna hear you – c’mon,’

His words go straight to your groin, you give up on control and slam into him, bruising your hipbones. Each thrust reminds you just how wet he is, the sound spurning you on.

You clench your teeth, and grind down on the vibrator, so close, so close. 

Finally the coil tightens in your body, ‘Oh fuck, fuck, Sans I’m-’ the heat hits you, throbbing deep between your legs. 

‘fuck yes, _____’ he groans, hand running over body, waist and hips, grasping at your skin.

You begin to slow, ‘_____, i – i need, i still need – fucking please-!’ You ignore your exhaustion.

Almost there. ‘Mmm fuckking hell, yes,’ you growl, ‘Come for me Sans, fucking come for me.’ 

You ignore your sensitive clit and press down on the vibrator. He can’t form words, body convulsing, trying so hard to come again. ‘That’s it baby boy, come on - I’ve got you, don’t force it, I know you wanna come again, it’ll happen, that’s it, that’s it-’ you ramble, lost in your objective.

He cries out and clutches your back, cunt pressed desperately against yours, vibrator searing through his clit.

You keep it up, ignoring the sweat running down your body, ignoring the pain in your hips, until he finally lets out something between a growl and a scream, phalanges nearly ripping your shirt, neck tilted back so far his forehead touches the pillow.

You let him ride it out, moving until his tone turns desperate, ‘t-too much, too much, it’s-’

‘Okay, s’okay, I’m stopping, stopping.’

You slow to a halt, gasping for air as if you’ve just run a triathlon. Body shaking, you kiss his shoulder, ‘m’gonna pull out.’ He looks up at you, eyelights hazy, and nods.

He shivers as you pull out slowly, cum dribbling from his cunt, the milky white substance covering your dildo. The sight makes your mouth water.

You fumble with the harness and manage to pull it off, your own cum staining the inside. You set it who knows where and fall back into bed, body sore, but completely satisfied.

He struggles to catch his breath, arm over his eye sockets, sweat sliding down his forehead. 

‘You okay?’ You breathe.

He lifts his arm to look at you, answers with what sounds like a yes, and reaches for you. You settle around him, caught in a mixture of exhaustion and euphoria.

‘fucking amazing,’ he slurs against your body, on the precipice of sleep.

'Mhmm,' you articulate, before joining him.

-

Epilogue:

‘…did i do it again? ah shit, i’m sorry.’

You shrug, 'No one'll see it. Sides' you know I don't mind it.’ You twist over your shoulder, trying and failing to see the scratch marks he's left all over your back.

‘you're bleeding.’

‘It's scabbed over, and it's surface level. Besides,' you place a hand on your hip faking conceit, 'Tells me I did a good job.' He rolls his eyelights, but the corner of his mouth twitches upward.

‘My back isn’t the problem. The problem,’ you groan, ‘is everything else.’ Your lower body aches, tendons stretched tight, muscles sore, skin bruised.

‘yeah, uh, sorry bout that,’ he shoves his hands into his jacket pockets forgetting he's in boxers, hands awkwardly hovering in midair with nothing to hide in. 

‘Not your fault. I kind of asked for it.’

‘actually, i asked for it... like five times.’ He says, voice sheepish.

‘It was three times. And you weren’t the only one.’

‘i guess.’

‘So, would you be up for doing it again?’

He gives you a skeptical look. ‘not anytime soon – that would be a real awkward ER visit.’

‘Oh c'mon, it's not that bad. But yeah, okay. Not anytime soon. I’ll wait till these heal,’ you glance at the layer of fat over your pubic bone. ‘Didn’t even know I could bruise there…’ you mumble to yourself.

**Author's Note:**

> I'm a little on the fence about this one. Something about it bugs me, but I don't know what. Constructive criticism is welcomed.
> 
> This is a tentative sequel to 'I'm Bi-Partisan To You.' But it can be read separately, no context needed.


End file.
